// Buffer cache.
//
// The buffer cache is a linked list of buf structures holding
// cached copies of disk block contents.  Caching disk blocks
// in memory reduces the number of disk reads and also provides
// a synchronization point for disk blocks used by multiple processes.
//
// Interface:
// * To get a buffer for a particular disk block, call bread.
// * After changing buffer data, call bwrite to write it to disk.
// * When done with the buffer, call brelse.
// * Do not use the buffer after calling brelse.
// * Only one process at a time can use a buffer,
//     so do not keep them longer than necessary.
//
// The implementation uses three state flags internally:
// * B_BUSY: the block has been returned from bread
//     and has not been passed back to brelse.
// * B_VALID: the buffer data has been read from the disk.
// * B_DIRTY: the buffer data has been modified
//     and needs to be written to disk.

#include "fs.h"

struct {
    struct spinlock lock;
    struct buf buf[NBUF];

    // Linked list of all buffers, through prev/next.
    // head.next is most recently used.
    struct buf head;
} bcache;

void binit(void)
{
    struct buf *b;

    initlock(&bcache.lock, "bcache");

    // Create linked list of buffers
    bcache.head.prev = &bcache.head;
    bcache.head.next = &bcache.head;
    for (b = bcache.buf; b < bcache.buf + NBUF; b++) {
        b->next = bcache.head.next;
        b->prev = &bcache.head;
        b->dev = -1;
        bcache.head.next->prev = b;
        bcache.head.next = b;
    }
}

// Look through buffer cache for sector on device dev.
// If not found, allocate a buffer.
// In either case, return B_BUSY buffer.
static struct buf *bget(uint dev, uint sector)
{
    struct buf *b;

    acquire(&bcache.lock);

loop:
    // Is the sector already cached?
    for (b = bcache.head.next; b != &bcache.head; b = b->next) {
        if (b->dev == dev && b->sector == sector) {
            if (!(b->flags & B_BUSY)) {
                b->flags |= B_BUSY;
                release(&bcache.lock);
                return b;
            }
            sleep(b, &bcache.lock);
            goto loop;
        }
    }

    // Not cached; recycle some non-busy and clean buffer.
    // "clean" because B_DIRTY and !B_BUSY means log.c
    // hasn't yet committed the changes to the buffer.
    for (b = bcache.head.prev; b != &bcache.head; b = b->prev) {
        if ((b->flags & B_BUSY) == 0 && (b->flags & B_DIRTY) == 0) {
            b->dev = dev;
            b->sector = sector;
            b->flags = B_BUSY;
            release(&bcache.lock);
            return b;
        }
    }
    panic("bget: no buffers");
}

// Return a B_BUSY buf with the contents of the indicated disk sector.
struct buf *bread(uint dev, uint sector)
{
    struct buf *b;

    b = bget(dev, sector);
    if (!(b->flags & B_VALID))
        iderw(b);
    return b;
}

// Write b's contents to disk.  Must be B_BUSY.
void bwrite(struct buf *b)
{
    if ((b->flags & B_BUSY) == 0)
        panic("bwrite");
    b->flags |= B_DIRTY;
    iderw(b);
}

// Release a B_BUSY buffer.
// Move to the head of the MRU list.
void brelse(struct buf *b)
{
    if ((b->flags & B_BUSY) == 0)
        panic("brelse");

    acquire(&bcache.lock);

    b->next->prev = b->prev;
    b->prev->next = b->next;
    b->next = bcache.head.next;
    b->prev = &bcache.head;
    bcache.head.next->prev = b;
    bcache.head.next = b;

    b->flags &= ~B_BUSY;
    wakeup(b);

    release(&bcache.lock);
}
